Measurement of physiological pressures is of interest to both clinicians and researchers. Physiological pressure measurements obtained from laboratory animals provide researchers with valuable information regarding the safety and efficiency of pharmaceutical agents, and the toxicity of chemicals, and leads to better understanding of human physiology. Physiological pressure measurements also have human clinical values, such as providing diagnostic information, assessing the safety and efficiency of drugs in clinical trials, and controlling implantable medical devices, such as pacemakers.
Arterial blood pressure is of particular interest to the researcher or clinician, because arterial blood pressure fluctuates over time in response to various conditions, such as an injection of pharmaceutical agent or chemical, or the activity level of an animal being observed. Arterial blood pressure fluctuations, however, often make it necessary to obtain chronic, frequent measurements to identify the effect of the injected pharmaceutical agent or chemical, or to properly control an implantable medical device.
In addition to arterial blood pressure, other pressure measurements are also of interest, such as venous pressure, pulmonary pressure, intracranial pressure, bladder pressure, intrauterine pressure, gastro-intestinal pressure, and other physiological pressures. For example, intrapleural or blood pressure can be used to determine the rate of respiration in addition to providing general information related to respiratory function. Measurements of intracranial pressure from laboratory animals are often used to project which methods of treatment and management are most effective in humans.
Chronic measurement of physiological pressures provides vital information for clinical care of humans. Patients with high blood pressure could benefit from an implantable device which could chronically monitor pressure as a means of determining optimal dosage for a drug or biofeedback therapy. Such a device could also be used as a means of providing feedback to a closed-loop drug delivery system for controlling blood pressure, or to a cardiac pacemaker as a means of optimizing pacing control parameters.
Infants who have been identified as being at risk for sudden infant death syndrome could also benefit. It is desirable to monitor changes in intrapleural pressure as a reliable measurement of respiratory rate in these infants by means which would allow the infant to roll and move freely about its crib without being restrained by wires extending from a vest.
Chronic monitoring of intracranial pressure is also important for infants with hydrocephalitis and patients with head injury. Hydrocephalitis and head injuries can cause excessive pressure buildup within the brain, resulting in death or serious brain damage. In most cases, corrective action can be taken if the buildup of pressure can be quickly detected.
This need to obtain accurate and ongoing physiological pressure measurements within various parts of animals and humans is discussed in detail in the Brockway U.S. Pat. No. 4,846,191 assigned the assignee of the present application, and which is herein incorporated by reference. The Brockway et al. `191 patent discloses a pressure measurement device for monitoring physiological pressures, such as blood pressure, in various locations in an animal or human. The pressure measurement device utilizes a fluid-filled pressure transmission catheter (PTC) with a gel membrane located at a tip of the PTC. The tip of the PTC is positioned in an area where physiological pressure is to be measured. The PTC extends from a small implantable housing that contains a transducer, signal-processing and telemetry circuitry, and a battery. The fluid-filled PTC communicates the pressure from the area where pressure is to be measured to the transducer within the housing, which generates an electrical pressure signal representing the communicated pressure. The signal-processing and telemetry circuitry in the housing receives the pressure signal generated by the transducer and provides a telemetry signal representing the pressure signal. The signal-processing and telemetry circuitry transmits the telemetry signal to a receiver which is external to the animal or human.
In some applications of the pressure measurement device disclosed in the Brockway et al. `191 patent, the housing cannot be implanted within close proximity to the area where pressure is to be measured due to physical limitations and practical considerations of surgical procedures. When the housing is not within close proximity to the area where pressure is to be measured, the length of the catheter that is required may be too long to assure that errors, resulting from decreased dynamic response or changes in posture, be within acceptable limits for the given application. For example, if the vertical distance from the PTC tip to the transducer changes due to posture, an error in the pressure measurement occurs. Every one centimeter change in vertical distance creates approximately one millimeter Hg error in the pressure measurement for one preferred low-viscosity fluid used in the catheter. This pressure measurement error is known as "head pressure artifact" and is very significant in certain applications. Furthermore, as the length of the PTC increases, the dynamic response of the pressure measurement device is reduced. In certain applications, the required length of the PTC is so long that a sufficient dynamic response cannot be obtained.
In addition, the Brockway et al. `191 patent does not disclose a pre-compensated, disposable, and easily replaceable transducer. Rather, since the transducer is inside the housing, if the transducer disclosed in the Brockway et al. `191 patent fails, the entire pressure-sensing device must be returned to the manufacturer for replacement to ensure proper compensation, mounting of the transducer, and sealing of the implant body.
For reasons stated above and for other reasons presented in greater detail in the Description of the Preferred Embodiments section of the present specification, there is a need for a pressure measurement device that is capable of measuring pressures in more animal and human applications, with better dynamic response, and with more accurate pressure measurements than currently possible with present pressure measurement devices. In addition, it is desired that the transducer and catheter of the pressure measurement device be more easily replaceable than currently possible with present pressure measurement devices.